The present invention relates to an interferometer for areally measuring an optically smooth surface and to a method for areally measuring an optically smooth surface. An interferometer of this kind is also known as a Tilted Wave Interferometer (TWI). The technical basis for the present invention is a method invented at the Institute of Applied Optics (Institut fũr Technische Optik-ITO) at Stuttgart University for measuring the surface of aspherical and freeform surfaces. It uses a set of mutually tilted wave fronts in order to locally compensate for the specimen's deviation from the sphere of best fit. The set generally consists of a plurality of (e.g. 49) wave fronts that are tilted relative to one another, and fundamentally differs from other interferometers which use just two tilted wave fronts in order to produce two wave fronts that are polarized orthogonally to one another for example. An embodiment, by way of example, is described in DE 10 2006 057 606 B4. The Tilted Wave Interferometer presented therein comprises a point light source array (PLSA) and an optical system that consists of a collimator, an optional interferometer objective, a beam splitter, an interferometer stop, imaging optics and a camera and that is arranged downstream of the PLSA within the optical path of light emanating from the PLSA, the interferometer stop being arranged approximately within the object-side focal plane of the imaging optics, which plane is also known as the Fourier plane.
Further TWIs are known from the publication W. Osten et al., Testing aspheric lenses: New approaches, Optoelectronics, Instruments and Data Processing, Vol. 46, No. 4, 1 Aug. 2010, pages 329-339, XP055337077, US ISSN: 8756-6990, DOI: 10.3103/S8756699010040059, from CN 103 759 668 A and from CN 102 607 454 A.
Known embodiments of the TWI produce their interferograms by superimposing the object wave fronts reflected by the specimen onto a reference wave that is coherent with all the object wave fronts generated by beam splitting from the light from a light source before the tilted object wave fronts are produced. In the embodiment known from DE 10 2006 057 606 B4, this reference wave is guided separately and coupled back in by the beam splitter only after the object wave fronts reflected by the specimen have passed the collimator, such that evaluable interference fringes form on the camera, from which fringes the form deviation of the specimen can be deduced using the method described in DE 10 2006 057 606 B4.
The separate coupling of the reference wave front results in significant differences in the optical paths of the reference wave and the object waves. This results, for example due to thermally induced changes in the structure or local fluctuations in the refractive index of the air, in a disadvantageous instability of the phase differences between the reference wave and the object waves.
The Fizeau interferometers, which have been known since the 19th century and are very widely used due to the stability thereof, produce the reference wave by means of beam-splitting Fizeau surface, the surface normals of which are approximately perpendicular to those of the incident wave front. Since this partially reflective Fizeau surface is usually the last surface before the specimen, for example in a Fizeau objective (transmission sphere), and the beams in the test configuration usually used, known as the “null test”, usually strike the specimen perpendicularly and are thus reflected back into themselves, reference and object wave fronts are separated only over a short distance between the Fizeau surface and the specimen and otherwise pass through the interferometer along virtually identical paths. The Fizeau interferometer is therefore also known as a “common path” interferometer. Since only the difference between the reference wave front and the object wave front is imaged in the interferogram, the property of the common paths has a positive effect on the reproducibility of the measurement results.
In the Tilted Wave Interferometer known from DE 10 2006 057 606 B4, a phase shifting method was used to evaluate the interferograms, the reference wave being shifted by a piezo actuator, in a plurality of steps, and an image stack being sequentially captured. Due to the sequential operating mode, the phase shifting leads to a long measuring time. Moreover, methods of this kind are susceptible to oscillations, which makes it more difficult to use the interferometer in manufacturing for example.
Further TWIs are known from the publication W. Osten et al., Testing aspheric lenses: New approaches, Optoelectronics, Instruments and Data Processing, Vol. 46, No. 4, 1 Aug. 2010, pages 329-339, XP055337077, US ISSN: 8756-6990, DOI: 10.3103/S8756699010040059, from CN 103 759 668 A and from CN 102 607 454 A. The device aspects of the present invention differ from the prior art according to CN 102 607 454 A by the characterizing features of claim 1, and the method aspects thereof differ by the characterizing features of the independent method claim.
Producing a sample-specific reference wave for each sample prevents waves that emanate from a plurality of Fizeau reflexes from being superimposed, in a disturbing manner, on the detector. In contrast, in the case of a TWI according to CN 102 607 454 A that uses a Fizeau objective in conjunction with a reference wave that is always the same for each measured value recording, there is likely to be disturbing, disadvantageous superimposition with waves emanating from a plurality of Fizeau reflexes.
The fact that the interferometer is designed to illuminate the optically smooth surface, during the measurement thereof, with different samples of mutually differing object waves, each sample producing its own reference wave that is deactivated when illumination with a different sample occurs, prevents this problem of the disturbing reflexes. Furthermore, the following advantages result in comparison with DE 10 2006 057 606 B4: it is not necessary to use an interferometer objective that has been manufactured individually for this interferometer. The Fizeau objectives that can be used in the invention are widely commercially available and accordingly cheaper than such custom-built models.
Using a Fizeau objective as the interferometer objective makes it possible to use a reflection of the object wave, appearing at the partially reflective surface of the Fizeau objective, as the reference wave front source. When the reference wave is produced in this way, with the exception of the path from the Fizeau surface to the specimen, the optical paths for the object waves and reference waves that extend within the interferometer according to the invention differ only slightly (common path design). This results in advantages in terms of the calibration stability and in terms of the sensitivity of the interferometer to fluctuations in the refractive index of the air, or other influences such as thermal deformation of the structure, or oscillations. The stability of the phase difference between the object wave and the reference wave, which stability is improved by the common path design, opens up new possibilities for evaluating the interferograms.
The design of the interferometer is more compact overall, and consists of fewer optical components than the interferometer from DE 10 2006 057 606 B4.
Phase shifting methods for evaluating the phase are still possible in the case of the interferometer, but, as an alternative option, it is also possible to use carrier frequency methods for determining the phase, and this is advantageous in particular situations.
In the known Tilted Wave Interferometer method, the specimen is illuminated by different samples A1 . . . AN of object waves, and an interferogram is recorded in each case. A typical size for the number N of samples used in a Tilted Wave Interferometer is 4. The measurement data recording using said 4 samples of object wave fronts prevents the object wave fronts that impinge on the detector from overlapping and it thus no longer being possible to evaluate the interferograms. Since the reference wave is supplied separately, said wave illuminates the detector in the case of all recordings using the samples of object waves, and thus results in the desired interferograms.
Replacing the specific interferometer objective used in the Tilted Wave Interferometer known from DE 10 2006 057 606 B4 with a Fizeau lens would initially cause the Fizeau surface to produce reflexes which, during normal operation of the Tilted Wave Interferometer, cannot be used as the reference wave but would instead cause interference. However, sample A1, for example, would contain an object wave front that would produce a reflex on the Fizeau surface that would be used as the reference wave front. For example, in a known embodiment, this would be an object wave front that is produced on the optical axis by means of a point light source and is propagated, untilted, in parallel with the optical axis. However, as soon as a different sample A2 . . . AN is introduced, said wave front is inherently deactivated and therefore cannot produce a reference wave front.